Electric Communication: Functions in the Social Behavior of Eigenmannia Virescens
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ELECTRIC COMMUNICATION: FUNCTIONS IN THE SOCIAL BEHAVIOR OF EIGENMANNIA VIRESCENS by CARL D. HOPKINS 1) 2) (The Rockefeller University, New York, N.Y., USA) (With 10 Figures) (Rec. 15-VI-1973) Electric communication has evolved in certain fishes for the purpose of identification, for forming or maintaining social relationships, and for syn- chronizing behavior. Relatively little is known regarding this communication channel. Among several distantly related groups of fishes, electric signals originate in specialized electric organs, are transmitted as electric currents in water, and are received by specialized cutaneous electroreceptors. Two of the principal groups of fishes that are thought to use electrical communication, the Gymnotoidei and the Mormyriformes, have members that produce two distinct classes of electric discharges which I refer to as "tone discharges" and "pulse discharges." All members of a given species appear to employ one type of discharge or the other. Tone discharges are those in which the individual impulses of the electric organ have a long duration in comparison to the interval between successive impulses, and in which the frequency of the discharge is very stable. Pulse discharges are those in which individual impulses are brief with respect to the interval between impulses, and in which the frequency is variable. The electric organ and 1) I am especially grateful to Professor PETERMARLER for his criticism, suggestions, and help with every phase of this work. I am grateful for invaluable help in the field work from JOSÉand SUSANTORRE-BUENO, R. HAVENWILEY, PETER MARLER, ANDREA HOPKINS,DAVID SINGH, EMERICH ROTH, and my wife, KATHRYNHOPKINS. I also thank MICHECANGELOROSSETTO for design and assistance with electronic equipment, RICHARD SOARESand CATEDOLAN for careful typing of field notes, and SHERYLPHILIPS for assistance in preparing spectograms. I thank ROGERPAYNE, DONALD GRIFFIN, MICHAEL V. L. BENNETTand WALTERHEILIGENBERG for their criticisms of various drafts of this paper. I thank Professor T. H. BULLOCKfor support during the preparation of the manuscript. This study was carried out under the auspices of the Ministry of Agriculture and Natural Resources in Guyana. I was supported by a training grant from the National Institute of Health (GM 01789). 2) Present address: Department of Ecology and Behavioral Biology, University of Minnesota, Minneapolis, Minnesota 55455, USA. 271 electroreceptors of these fish are also employed in detection of objects (LISSMAN & MACHIN, 1958). Studies of electric communication have dealt with only a few species of electric fish ?BLTLLOCK, WESTBY & Box, ig7o; MOLLER, 1970; ]BLACK-CLEWORTFI,I970; VALONE, 1970; HOPKINS, 1972 a & b) and the majority of these studies have dealt with species that produce pulse dis- charges. This paper describes the use of electric communication in the social behavior of one of the tone-discharging South American gymnotids, Eigen- iiiannia virescens. METHODS The gymnotids are shy, nocturnal fishes; thus extended visual observations are possible only with the use of aquaria. The majority of this study is based on observations conducted in aquaria in Guyana, South America, in an area where Eigenmannia virescens is relatively common. The study site was located near Moco-moco Creek, a tributary of the Takutu River (Amazon Drainage), located at the base of the Kanuku Mountains in the Rupununi District of Guyana (3° 19' N, 59° 39' W). Specimens were transported immediately to io to 20 liter containers and held in isolation. A specimen was never kept for observations for longer than 12 days (average = 5 days) and they were not fed during their period of captivity. I made observations at night on fish in either a 75 or i io liter aquarium, illuminated from behind by the light of a single candle surrounded with red cellophane. Eigenmannia did not appear to be disturbed by this dim, red illumination. The fish were usually placed together for interactions in pairs. Eigenmannia were individually identified by taking careful note of small scars and deformities as well as natural color patterns on the skin. For example, one individual had approximately 2 cm of its tail regenerating; another had a small notch in its anal fin 3 mm from the end. Since the fish were usually isolated except for observations in small groups, the identity of an individual was rarely confused. At the completion of observations on a group of fish, the individuals were sexed and preserved in formalin. The electric signals from the fish were detected with two wire electrodes buried under the gravel of the aquarium. After the potential difference between the tw-o electrodes was amplified with a conventional audio amplifier, the signals passed into a tape recorder with a loud speaker monitor. Since the electric discharges of gymnotids consist of brief pulses similar to nerve spikes, the discharges were audible once they had been transduced by the loud speaker. The tape recorder (Uher Report Stereo 4200 or 4400) had two recording channels, one for fish discharges and one for voice commentary. For most recordings, I used a tape speed of 3.% i.p.s. I analyzed tape-recorded signals sound spectrographically using either the Kay Electric Sound Spectrograph (Model 7029-A) or the Federal Scientific UA-7B Ubiquitous Spectrum Analyzer. The Federal Scientific spectrum analyzer had the advantage of operating in "real time", but since it has rarely been used in biological research, its output had to be modified to produce traditional spectrograms. These modifications are discussed in HOPKINS,ROSSETTO & LUTJEN (1974). The continuous and steady tone discharge of Eigenmannia appears as a series of dark bands on the spectrogram. Individuals tended to differ slightly in their discharge frequency, thus producing different bands that could be used to individually identify the signaler. I was thus able to determine the identity of the individual producing any time-varying frequency change. Br?.4cx-CLEwoRTH(1970), HOPKINS(I972a) and others have found that variations in frequency are important for electrical communication in fish. .